Owing to their construction, the scraper chain conveyors used in underground coal mining are subject to "chain sag" involving a slackening of the chain in the region of the main drive and/or auxiliary drive after the chain has passed round the sprocket wheel. The sag is due partly to the fact that the conveyors are constructed in individual segments, which necessitates a certain amount of play in the chain, and partly to elongation of the chain during its operating life, and variations in the length of the conveyor itself.
At the auxiliary drive the sag occurs on the open, exposed upper side of the conveyor, where the untensioned slack portion of the chain is unconfined, so that sag in this region of the conveyor seldom causes any problems. At the main drive, however, sag occurs in the concealed lower part of the main drive machine frame, where clearances are limited, so that the slack may cause jamming and severe damage to the chain and/or to the machine frame.
In the conveyors normally used hitherto, the main and auxiliary drives are usually controlled so that they share the instantaneous conveying load in equal proportions. This distribution of the load ensures that when the conveyor is operating in the high load range, in which the tension in the active upper strand is distinctly higher than in the returning lower strand, the lower strand in the lower part of the machine frame is always tensioned, and sag can only occur at the auxiliary drive, in the visible upper part of the conveyor.
However, if the conveyor is running under no load or under low load, the tension required for the lower strand is greater than the tension required for the upper strand, since the lower strand is subject to increased friction due to small coal and dirt which accumulates in the lower part of the machine frame. The consequence is that, even with driving output symmetrically divided between the main and auxiliary drives, sag will occur first at the main drive, where it may be unnoticed and may quickly lead to severe damage to the conveyor. If the conveyor is provided with more main drives than auxiliary drives, and all drives receive the same output, the risk of sag at the main drives is even greater.